Conventionally, an electric double-layer capacitor comprises a pair of electrodes, a separator provided between the electrodes, and collection layers of each electrode. In typical electrodes used for the electric double-layer capacitor, a carbon material such as carbon powder and a fibrous carbon is used.
As a manufacturing method of the electrode used for the electric double-layer capacitor, a method for adding a conductive material such as acetylene black, and a resin such as polytetrafluoroethylene and tetrafluoroethylene resin as a binder, to activated carbon powder which is a typical material of the electrode and mixing the resultant before pressure-molding to form a sheet-like polarized electrode is known. Moreover, a method for adding the mixture to a solvent and applying the resultant to a current collector (Coating method) is also suited.
Such an electric double-layer capacitor has a problem of reduction in capacitance when left under high temperature environment, which is seemed to be caused by a reaction of functional groups on a surface of the activated carbon. Although various proposals have been made to solve this problem, the results are not sufficient. In contrast, since a carbon nanotube has few such functional groups, it is known that its lifespan property is better than that of the activated carbon.
Thus, for increasing the capacitance, there is an attempt to use a sheet-like polarized electrode formed by mixing the activated carbon, the carbon nanotube, and the resin-based binder before pressure-molding (see for example, JP 2001237149 A and JP 2000124079 A).
In such an electrode for the electric double-layer capacitor, the resin-based binder is used when pressure-molding and applying a mixed solution of activated carbon powder and the like to the current collector. However, in an aspect of reduction in resistance of the electrode, since the resin-based binder acts as an impurity, there is a problem that the electrode obtained by using the resin-based binder has high resistance.